1. Field of the Invention
The present invention relates to a tone for a developer, which develops latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like. The present invention also relates to an image-forming apparatus and image-forming process using the toner. Specifically, the present invention relates to a toner, developer, developing apparatus for electrophotography, which are used for copiers, laser printers, facsimiles for plain paper and the like using a direct or indirect electrophotographic developing system. Further, the present invention is directed to a toner for electrophotography, an image-forming apparatus (developing apparatus) and a process cartridge which are used for full-color copiers, full-color laser printers, full-color plain paper facsimiles and the like using a direct or indirect electrophotographic multicolor developing system.
2. Description of the Related Art
In electrophotography, electrostatic recording, electrostatic printing and the like, a developer is, for example, applied to an latent electrostatic image bearing member, e.g., a photoconductor or the like, so as to dispose the developer onto a latent electrostatic image formed on the latent electrostatic image bearing member in a developing step, the developer disposed on the image is transferred to a recording medium, e.g., a recording paper, or the like in a transferring step, thereafter the transferred developer is fixed on the recording medium in a fixing step. As such developer used for developing the latent electrostatic image formed on the latent electrostatic image, it is commonly known to use single-component developers such as magnetic toner and non-magnetic toner, which do not require a double-component developer consisting of a carrier and a toner, or a carrier. As dry toners used in electrophotography, electrostatic recording, electrostatic printing or the like, conventional toners are one which is formed by melting and kneading a toner binder (binder resin) such as styrene resin, polyester or the like, colorants, and the like together, then pulverizing the mixture.
These dry toners are, after used for developing and transferred on a sheet of paper or the like, fixed on the sheet by heating and melting the toner using a heat roller. If a temperature of the heat roller is overly high, in this procedure, a problem, so called “offset” occurs. Offset is the problem that the toner is excessively melted and adhered onto the heat roller. If a temperature of the heat roller is overly low, on the other hand, a problem arises in that a degree of melting the toner is insufficient, resulted in insufficient fixing. Accordingly, there are demands in a toner having a higher temperature to cause offset (excellent hot offset resistance) and a low fixing temperature (excellent fixing ability at low temperature), while energy conservation, miniaturization of apparatuses such as copiers are considered on the other hand. Toners also require a heat-resistant storability that suppresses blocking of toner when the toner is stored, and at a temperature of atmosphere within the apparatus where the toner is accommodated. Especially, low melting viscosity of toner is essential in full-color copiers, full-color printers in order to obtain gloss and color mixture of an image. As a consequence, polyester toner binders exhibiting sharp melting has been used in such toner. However, this toner tends to cause offset. To prevent offset, in apparatuses for full-color, silicone oil or the like has conventionally been applied on the heat roller. Yet, the method of applying silicone oil or the like to the heat roller has a problem in that the apparatuses need to equip an oil tank and an oil applier, therefore the apparatuses become more complex in their structures and large in their size. It also leads a deterioration of the heat roller, so maintenance is required at every arbitrary term. Further, it is unavoidable that the oil is attached to copier paper, films for OHP (over head projector) and the like, and especially with the films for OHP, there is a problem in that the attached oil cases deterioration in color tone.
Owing to prevent a toner fusion without applying an oil to a heat roller, wax is added to a toner. In this method, however, mold-releasing effect is largely affected from a condition of dispersed wax within a toner binder. Wax does not exhibit its mold-releasing ability if the wax is compatible with a toner binder. Wax exhibits its mold-releasing ability and improves mold-releasing ability of toner when the wax stays within a toner binder as incompatible domain particles. If a diameter of domain particles is excessively big, there is a problem in that excellent images cannot be obtained. This is caused by an increase in a ratio of wax existed in a surface portion of a toner with respect to other component of the toner, related to an increase in the diameter thereof. Therefore, the toner exhibits an aggregation hence impair fluidity. Moreover, filming occurs where wax migrates to a carrier or a photoconductor during long-term use. Yet there is a problem in that color reproducibility and clearness of an image are impaired in the case of color toner. On the contrary, if a diameter of the domain particle is overly small, the wax is excessively dispersed so that sufficient mould-releasing ability cannot be obtained. Although it is necessary to control a diameter of wax as mentioned above, an appropriate method thereof has not been found yet. For example, in the case of toners manufactured by pulverization, control of wax diameter largely relies upon shear force of mixing during melting and kneading. Polyester resin, which has commonly been used for a toner binder, has a low viscosity hence sufficient shear force cannot be added thereto. Consequently, it is very difficult to control distribution of wax and to obtain a suitable diameter especially for these toners.
Another problem of pulverization is that more wax is exposed to a surface of toner since the wax tends to have broken edges in its shape.
Although improvement of toners has been attempted by miniaturize a diameter of toner particle or narrowing particle diameter distribution of toner in order to obtain high quality images, uniform particle shape cannot be obtained by ordinary manufacturing methods of mixing pulverization. Moreover, the toner is still pulverized so that overly fine toner particles are generated, in a course of mixing with carrier in a developing member of the apparatus, or, by a contact stresses between a developing roller, and a toner applying roller, a layer thickness controlling blade, or a friction charging blade. These lead deterioration of image quality. In addition, a superplasticizer embedded on the surface of toner also leads deterioration of image quality. Further, fluidity of the toner particles is insufficient because of shapes of the toner particle, and thus a large amount of the superplasticizer is required or a filling factor of the toner into a toner bottle becomes low. Accordingly, these are considered to be factors that inhibit a miniaturization of apparatuses.
Yet there is a problem in that missing portions can be found in the transferred image and an amount of the toner consumption becomes large to cover the missing portions in the transferred image. This is due to the impaired transferring ability caused by that a process of transferring, in which an image formed by multicolor toner is transferred to a recording medium or a sheet of paper, is complicated in order to form full-color image, and the toner has non-uninformed particle diameter, such as in a case of a pulverized toner.
Accordingly, a strong demand has arisen in obtaining high quality images which do not have any missing part by further improving transfer efficiency leading to a reduction in an amount of toner consumption, and also reducing running cost. If transfer efficiency is remarkably excellent, a cleaning unit, which removes remained toner on a photoconductor or a transfer after transferring, can be omitted form an apparatus. Therefore, the apparatus can be miniaturized and low const thereof can be achieved together with having a merit of reducing a disposed toner. Hence, various methods for manufacturing a spherical toner have been suggested in order to overcome the defects caused by a non-uniformly shaped toner.
Hitherto, extensive studies have been done to improve a quality of toner. It has been known that a mold-releasing agent (wax) having a low melting point, e.g., polyolefine, is added to a toner in order to improve fixing ability at low temperature and offset resistance. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 6-295093, 7-84401, and 9-258471 disclose toners that contain wax having a certain endothermic peak by DSC (differential scanning calorimetry). However, the toners disclosed the above patent publications still need to improve fixing ability at low temperature, offset resistance and also developing ability.
Also, JP-A Nos. 5-341577, 6-123999, 6-230600, and 6-324514 disclose candelilla wax, higher fatty acid wax, higher alcohol wax, vegetable wax (carnauba, rice), montan ester wax and the like as a mold-releasing agent of toner. However, the toners disclosed in the above patent publications still need to improve developing ability (charging ability) and durability. If the mould releasing agent having a low softening point is added to a toner, fluidity of the toner is decreased hence developing ability or transferring ability is also decreased. Moreover, charging ability, durability and storability of the toner prone to be affected thereby.
JP-A Nos. 11-258934, 11-258935, 4-299357, 4-337737, 6-208244, and 7-281478 disclose toners which contain two or more mold-releasing agents in order to enlarge a fixing region (non offset region). However these toners still have a problem in uniform dispersion of the mold-releasing agents within a toner particle.
JP-A No. 8-166686 discloses a toner which contains polyester resin and two types of offset inhibitors each having different acid values and softening points. However, these toners still have problem in developing ability. Moreover, JP-A Nos. 8-328293, and 10-161335 disclose a toner that defines a dispersion diameter of wax within the toner particle. However, there is a case that sufficient mold-releasing ability of the toner cannot be exhibited during fixing since a condition or positioning of the dispersed wax is not defined in the toner particle.
Furthermore, JP-A No. 2001-305782 discloses a toner in which spherical wax particles are fixed onto the surface of toner. However, the wax particles positioned on the surface of toner decreases fluidity of the toner and thus developing ability or transferring ability of the toner is also decreased. In addition, charging ability, durability, and storability of the toner also prone to be affected. JP-A No. 2002-6541 discloses a toner in which wax is included in the toner particle and the wax is located in the surface portion of the toner particle. However, there is a case that all of offset resistance, storability, and durability of the toner are insufficient.
Generally, a toner is manufactured by methods of kneading pulverization in which a thermoplastic resin is melted and mixed together with a colorant, and a mold-releasing agent or a charge control agent may further added depending on the cases, so as to make a mixture, and then the mixture is pulverized and graded. Further, depending on the cases, inorganic or organic fine particles are added onto the surface of toner particle in order to improve fluidity or cleaning ability. In ordinal methods of kneading pulverization, a shape and surface structure of toner particle are non-uniformed. Although it depends on crushability of materials and conditions of pulverizing step, it is not easy to control a shape and surface structure of toner particle arbitrarily. Also, it is difficult to narrow particle diameter distribution of a toner due to a limitation of grading performance and an increased cost thereby. Regarding a pulverized toner, it is a great task to control an average particle diameter of toner particle diameter distribution to a small particle diameter, especially 6 μm or less, from the viewpoint of yield, productivity and cost.
Along with wide spread of electrophotographic technology, not only a demand for obtaining more detailed and high quality image formation, but also a demand for high-speed, small lightweight, simple handling apparatus and multimode copy with a small number of sheet have arisen. As electrophotographic apparatuses has been more commonly applied for home use or personal use, not only office use, there is a demand especially in small light weight of apparatus, simplification of handling when copying with small number of sheets. In an image-forming apparatus using electrophotographic technology, a charger, a light irradiator and an image developer, which are disposed around a photoconductor, sequentially perform charging, light irradiating, and developing onto the photoconductor. When the charger performs charging, the image developer is already raised and starts an operation required for sufficiently charging a developer or the like, so as to ready for an operation of developing. This operating condition of the image developer is hold at a certain period so as to ready for a cycle of continuous copying. When a relatively small number of sheets of copying, i.e., one or a few sheets, is performed as in the case of home use or personal use, a toner contained in a developer is likely to have more chances to receive mechanical stress of stirring or the like. Further, a demand for a high-speed apparatus in recent years leads a rapid rise of apparatuses. This means that a toner has more chances to receive stronger mechanical stress by a rapid stirring with a short time or the like and an apparatus is more likely to receive an intensive thermal stress therein as a result of a rapid rise and high-speed operations of the apparatus.
In the case of a toner that is formed by kneading a resin and wax, the wax is included in the resin, and is stayed as fine particles. This is due to a reason that the wax generally shows poor compatibility with the resin. Another reason is that the wax exhibits mold-releasing ability only when the wax is stayed in the resin as incompatible domain particles, but the wax does not exhibit mold-releasing ability when the wax is compatible with the resin. However, the toner is easily grinded in smaller particles by mechanical stress cased by stirring or the like. The toner tends to be grinded in smaller particles when an intensive thermal stress is added to the toner, i.e., when the toner is rapidly heated. This is because the wax has a different coefficient of thermal expansion from that of the resin, generally the coefficient of thermal expansion of the wax (excluding microcrystalline wax) is 3% to 10% higher than the coefficient of thermal expansion of the resin since most types of wax tend to be rapidly crystallized through the state of amorphous. It is widely known that these overly grinded toner particles cause various inconveniences.
Apart from the inconveniences, there is also a problem when a developer containing a toner is used for a long term. In such developer, a consumption rate of toner particles varies depending on a diameter of the toner particle. Generally, a larger diameter of the toner particle tends to be consumed more rapidly than a smaller diameter of the toner particle. Therefore, a composition of the toner changes from the initial toner composition, and the composition of the toner may end up with having a large amount of small toner particles.
JP-A No. 2002-6541 is an excellent from the viewpoint that it discloses a necessity in that wax is included in a toner particle and the wax is located in a surface portion of the toner particle, in order to solve a problem of thermal stability during storage caused by the wax having a low melting point being located on a surface of the toner. However, this patent publication does not teach a problem of overly miniaturized toner particles by mechanical and thermal stresses added to the toner during the operations, and the undesirable relationship between the resin and the wax as mentioned above. The above patent publication does not suggest a manner to solve the problem neither.
For the purpose of positioning the wax in the surface portion of the toner particle in the technique disclosed in the above patent application, for example, the prepared slurry at 125° C. is quickly cooled down to 25° C. within ten seconds. Although an existing amount of wax in a toner is not mentioned therein, there is a description about the existing amount of wax only in examples which is showing that the existing amount of wax is less than 1% with respect to a toner particle. From this figure, it is easily imaged that the existing amount of wax is too small to exhibit mold-releasing effect, though it is attempted by bleeding the wax out to a surface of the fixing member so as to prevent a toner fusion thereon during fixing, and thus margin of hot offset is omitted.
As has been mentioned, wax included in a toner particle affects on a degree of toner fusion and fixing ability depending on a pulverizing state of the toner particle. However, it is difficult to control pulverization of toner particles and is further difficult to control a state of the included wax with foreseeing a state of pulverized toner particle.
If wax is not uniformly dispersed in a toner particle and only existed in a surface portion of the toner particle, an amount of the wax existed around the surface of the toner particle is large, and thus the toner particle prone to fuse on a carrier or the like. If wax is uniformly dispersed in a toner particle including a surface portion of the toner particle, on the other hand, a part of the surface portion of the toner particle where the wax does not exist becomes overly large, mold-releasing ability of the toner is insufficient during developing, transferring and cleaning compared with the expectation, and the toner particle prone to be crushed by mechanical or thermal stress. In addition, there is also a possibility that a characteristic of newly crushed fine toner particle is not desirable.